Semiconductor manufacturing, such as the fabrication of integrated circuits, typically entails the use of photolithography. A semiconductor substrate on which circuits are being formed, usually a silicon wafer, is coated with a material, such as a photoresist, that changes solubility when exposed to radiation. A lithography tool, such as a mask or reticle, positioned between the radiation source and the semiconductor substrate casts a shadow to control which areas of the substrate are exposed to the radiation. After the exposure, the photoresist is removed from either the exposed or the unexposed areas, leaving a patterned layer of photoresist on the wafer that protects parts of the wafer during a subsequent etching or diffusion process.
The photolithography process allows multiple integrated circuit devices or electromechanical devices, often referred to as “chips,” to be formed on each wafer. The wafer is then cut up into individual dies, each including a single integrated circuit device or electromechanical device. Ultimately, these dies are subjected to additional operations and packaged into individual integrated circuit chips or electromechanical devices.
During the manufacturing process, variations in exposure and focus necessitates that the patterns developed by lithographic processes be continually monitored or measured to determine if the dimensions of the patterns are within acceptable ranges. The importance of such monitoring, often referred to as process control, increases considerably as pattern sizes become smaller, especially as minimum feature sizes approach the limits of resolution available by the lithographic process. In order to achieve ever-higher device density, smaller and smaller feature sizes are required. This may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as corners and edges of various features. Features on the wafer are three-dimensional structures and a complete characterization must describe not just a surface dimension, such as the top width of a line or trench, but a complete three-dimensional profile of the feature. Process engineers must be able to accurately measure the critical dimensions (CD) of such surface features to fine tune the fabrication process and assure a desired device geometry is obtained.
Typically, CD measurements are made using instruments such as a scanning electron microscope (SEM). In a scanning electron microscope (SEM), a primary electron beam is focused to a fine spot that scans the surface to be observed. Secondary electrons are emitted from the surface as it is impacted by the primary beam. The secondary electrons are detected, and an image is formed, with the brightness at each point of the image being determined by the number of secondary electrons detected when the beam impacts a corresponding spot on the surface.
A focused ion beam (FIB) system is often used to expose a portion of a sample for observation. For example, the FIB can be used to mill a trench in a circuit to expose a vertical sidewall that displays a cross section showing the layers of the sample, such as a circuit or other structure having microscopic features.
As features continue to get smaller and smaller, however, there comes a point where the features to be measured are too small for the resolution provided by an ordinary SEM. Transmission electron microscopes (TEMs) allow observers to see extremely small features, on the order of nanometers. In contrast to SEMs, which only image the surface of a material, TEM also allows analysis of the internal structure of a sample. In a TEM, a broad beam impacts the sample and electrons that are transmitted through the sample are focused to form an image of the sample. The sample must be sufficiently thin to allow many of the electrons in the primary beam to travel though the sample and exit on the opposite site. Samples are typically less than 100 nm thick.
In a scanning transmission electron microscope (STEM), a primary electron beam is focused to a fine spot, and the spot is scanned across the sample surface. Electrons that are transmitted through the substrate are collected by an electron detector on the far side of the sample, and the intensity of each point on the image corresponds to the number of electrons collected as the primary beam impacts a corresponding point on the surface.
As semiconductor geometries continue to shrink, manufacturers increasingly rely on transmission electron microscopes (TEMs) for monitoring the process, analyzing defects, and investigating interface layer morphology. The term “TEM” as used herein refers to a TEM or a STEM, and references to preparing a sample for a TEM are to be understood to also include preparing a sample for viewing on an STEM.
Thin TEM samples cut from a bulk sample material are known as “lamellae.” Lamellae are typically less than 100 nm thick, but for some applications a lamella must be considerably thinner. With advanced semiconductor fabrication processes at 30 nm and below, a lamella needs to be less than 20 nm in thickness in order to avoid overlap among small-scale structures. Currently thinning below 60 nm is difficult and not robust. Thickness variations in the sample result in lamella bending, over-milling, or other catastrophic defects. For such thin samples, lamella preparation is a critical step in TEM analysis that significantly determines the quality of structural characterization and analysis of the smallest and most critical structures.
The use of focused ion beam (FIB) systems to create lamellae for TEM microscopy is known in the art. FIB systems are capable of milling lamellae sufficiently thin to be used in a TEM system. The use of dual-beam systems for TEM sample preparation is known in the art. A dual-beam system has a FIB column for milling a lamella from a bulk sample and a SEM column for imaging the lamella, typically as the lamella is being milled. Dual-beam systems improve the time required to prepare samples for TEM analysis. While the use of FIB methods in sample preparation has reduced the time required to prepare samples for TEM analysis down to only a few hours, it is not unusual to analyze 15 to 50 TEM samples from a given wafer. As a result, speed of sample preparation is a very important factor in the use of TEM analysis, especially for semiconductor process control.
FIGS. 1A and 1B show the preparation of a sample lamella for TEM analysis from a bulk sample material using a FIB. Bulk sample material 108 is loaded into sample stage and oriented so that its top surface is perpendicular to focused ion beam 104 emitted from a FIB column. A focused ion beam using a high beam current with a correspondingly large beam size is used to mill large amounts of material away from the front and back portion of the region of interest. The remaining material between the two milled rectangles 14 and 15 forms a thin vertical sample section 102 that includes an area of interest. After bulk thinning, the sample section is thinned (typically using progressively finer beam sizes and lower beam energy) until the desired thickness (typically less than 100 nm) is reached. Most of the ion beam machining done to create lamella 110 is performed with bulk sample material 108 and FIB column in this orientation.
Once the specimen reaches a desired thickness, the stage is typically tilted and a U-shaped cut is made at an angle partially along the bottom and sides of the sample section 102, leaving the sample hanging by tabs at either side at the top of the sample. The small tabs allow the least amount of material to be milled free after the sample is completely FIB polished, reducing the possibility of redeposition artifacts accumulating on the thin specimen. The sample section is then further thinned using progressively finer beam sizes. Finally, the tabs are cut to completely free the thinned lamella 110. After thinning the sample is freed from the bulk material at the sides and bottom, and the thinned TEM sample can be extracted.
Unfortunately, ultra thin lamellae formed using the prior art methods described above are subject to undesirable side effects known as “bending” and “curtaining.” When attempting to produce ultra thin samples (for example, 30 nm thickness or less) the sample may lose structural integrity and deform under forces acting on the sample, typically by bending or bowing toward one sample face or the other. If this occurs during or prior to a FIB thinning step, then the deformation of the region of interest toward or away from the beam may cause unacceptable damage to the sample.
Thickness variations caused by a milling artifact known as “curtaining” can also have a significant effect on TEM sample quality. When bulk sample material 108 is formed from a heterogeneous structure (e.g., metal gates and shields along with silicon and silicon dioxide), ion beam 104 preferentially mills the lighter elements at a higher mill rate. The heavier metal elements tend to shadow the lighter material underneath them. The resulting effect is a rippled face, which is not milled back as far in the areas of metal as it is milled in the areas without metal. FIG. 2 is a photomicrograph of a thinned TEM sample 102 showing curtaining on one sample face, in which the rippled features on the lamella face resemble a hanging curtain. Curtaining artifacts reduce the quality of the TEM imaging and limit the minimal useful specimen thickness. For ultra-thin TEM samples, the two cross-section faces are in very close proximity so thickness variations from curtaining effects can cause a sample lamella to be unusable. Thus, it is desirable to reduce curtaining artifacts during the preparation of TEM sample lamellae.
Curtaining and other artifact are also problems on cross section faces milled by a FIB for viewing with an SEM. Milling a hard material can result in “terracing,” that is, the edge rolls off in a series of terraces, rather than having a sharp vertical drop. FIG. 8 shows terracing caused by a hard layer. The terracing can cause curtaining artifacts and other artifacts to be formed below the terracing. Sample 800 includes a layer of aluminum oxide 802 over a layer of aluminum 804, which is softer than the oxide. A platinum protective layer 806 deposited over the aluminum oxide layer reduces the creation of milling artifacts, but the protective layer does not eliminate terracing. FIG. 8 shows the terraced edge 810 produced by the ion beam on the hard oxide layer. The terraced edge 810 of the oxide layer causes irregularities 812, such as curtaining artifacts, to be produced on the layer, such as aluminum layer 804, below the terracing.
FIG. 9 shows a scanning electron beam image of a sample 902 similar to that shown schematically in FIG. 8. A layer 904 of aluminum oxide sits over a layer of aluminum 906. A protective layer 908 is deposited over the oxide layer 904 to reduce the creation of artifacts. After the trench was milled to expose the cross section shown, the ion beam was scanned across the exposed face to mill a “cleaning cross section” using a current of about 180 nA. A “cleaning cross section” is typically a succession of advancing, serial line mills. The hard aluminum oxide layer shows terracing artifacts, which are difficult to observe in the black region in FIG. 9. The terracing in the hard oxide layer causes curtaining in the softer aluminum layer 906 below the aluminum oxide. Terracing and other uneven milling artifacts can also be produced in many materials when using high beam currents, such as from a plasma ion source.
Terraced artifacts can be difficult and time-consuming to prevent using prior art methods for reducing artifacts. Such methods include using reduced milling current and high beam overlap between scans in the final cleaning cross section. Some artifacts created when milling large cross sections are reduced by “rocking” the work piece, that is, alternating the ion beam impact angle, such as alternating the beam angle between plus 10 degrees and minus 10 degrees. Rocking does not reduce terracing artifacts, however. Terracing tends to create severe artifacts, such as severe curtaining, in the region below the terracing.
The most effective and widely proven alternative, backside milling, works reasonably well for TEM samples having a thickness of 50 to 100 nm, but for ultra-thin samples having a sample thickness of 30 nm or less, even samples prepared by backside milling often show milling artifacts resulting in an undesirably non-uniform sample face. Further, even for thicker samples, backside milling requires a liftout and inversion operation that is very time consuming. Current backside milling techniques are also performed manually, and are unsuitable for automation.
Thus, there is still a need for an improved method for the preparation of ultra-thin TEM samples that can reduce or eliminate bending and curtaining, and that is suitable for an automated sample preparation process.